This invention relates to a low pH coating composition for a recording medium especially adapted for ink jet printing, and method for making the same.
Ink jet printers employ a plurality of jets connected to a supply of liquid based ink. The jets have small openings and can be energized to emit uniform liquid droplets of ink in a continuous stream upon demand. The jets are controlled to print characters or images on a moving flat surface, usually a sheet of paper.
In order to improve print quality and drying time, many proposals have been made for coatings on paper to improve ink reception. For example, it is well known to coat paper with various absorptive pigments, binders and sizing agents. An aqueous suspension of these agents are applied to a paper or other web using conventional coating methods.
An ideal ink receiving coating on paper will allow rapid absorption of the liquid component of the ink, which correlates to a rapid ink drying time, while preventing diffusion of the ink colors into the paper substrate. At the same time, the coating should cause the ink pigment or dye to be fixed on the sheet surface in the form of well defined dots of uniform size and shape. The coating, upon drying, should be non-dusting and non-toxic. A correct balance of these properties is very difficult to achieve, especially at higher printer resolutions and smaller dot diameters.
While a variety of acceptable coatings can be devised in theory, it is also imperative for the sake of economy that the coatings are capable of being applied uniformly to a base sheet at a high rate of speed using conventional coating equipment. Many of the known absorptive pigments, such as those based on powdered forms of silica, cannot be employed because an excessive amount of binder is required for processing at the solids content necessary to achieve the desired minimum coat weight. The suspensions become too thick or dilatant to allow pumping and uniform application using a conventional paper coater such as a blade coater. If lower binder levels are employed, this may also result in excessive dusting in the finished product.
An ink receptive coating is applied to one or both sides of a substrate. The substrate may be optionally precoated with a size solution before the ink receptive coating is applied. The ink receptive coating is an aqueous suspension comprising silica pigment dispersed at low pH with a cationic fixing agent. Coatings with pH values of 4.0 to 7.5 have been found to give improved ink holdout over conventional coating compositions having pH values in the range of 8.0-9.0. The improved ink holdout results in better color saturation, reduced bleed through, and better overall print quality.
Silica gels are preferred pigments. Silica gels are commercially available that have acceptably low pH values and high void volumes desirable for ink absorption.
Cationized pigments hold more dye at the surface than anionic pigments. Accordingly, the coating composition of the invention preferably includes a cationic fixing agent, such as polycationic quaternary ammonium polymer, to cationize the pigment.
The coating composition of the invention includes a binder. Although several suitable binders could be used, low molecular weight, partially hydrolyzed polyvinyl alcohol is preferred.
Alumina may be added to the coating composition as a whitening agent and to improve rheology. For applications where an FDA approved coating is not required, a fluorescent whitening agent may be added.
Thus, broadly the coating composition of the invention has a pH value of 4.0 to 7.5, and comprises, by one dry parts by weight:
The preferred embodiment comprises, approximately, in bone dry parts by weight:
The coated ink jet medium of the present invention allows ink jet printing over a wide range of resolution with precise control of dot size (freedom from print mottle), dot size uniformity, and dot shape.
The coating composition of the invention comprises an aqueous suspension of an absorptive silica pigment, a binder and a cationic fixing agent, dispersed at an acidic pH. Although the precise technical effect is not fully known, it is believed that the acidic pH of the coating composition enhances the cationic function. The coating composition of the invention is applied to a substrate, for example paper. When an ink jet ink is applied to the coated paper, the absorptive silica particles absorb the ink solvent (water) leaving the dyes on the surface of the particles, and the cationic agent assists in fixing the dye on the surface. This results in rapid dry time and improved ink hold out and color density.
The pH value of the composition is between 4.0 and 7.5, but preferably in the range of 4.5 to 5.5. Coatings at these pH values have been found to have improved holdout of the ink jet ink dyes on the paper surface as compared to conventional alkaline coating compositions having pH values of 8.0 to 9.0. The improved holdout results in enhanced color saturation and better overall print quality. It also reduces dye penetration to the opposite side of the sheet. The technical cause of the improvements observed in the low pH coating compositions of the invention is not precisely known, however.
Tests have been conducted on coating formulations of varying pH values. Coating compositions were prepared in accordance with the invention using pigments and binders selected to result in low pH values. Acid was added to lower pH values. A base, sodium hydroxide, was added to increase pH. It was found that when the pH of the composition is reduced to a value below about 4.0 the composition becomes unstablexe2x80x94the ingredients become less compatible and viscosity increases with time. As the pH is increased above about 5.5, to beyond a pH value of 7.5, the dispersion becomes similarly unstable, the viscosity of the composition becoming undesirably high. Accordingly, it has been found that a pH value in the range of about 4.5 to about 5.5 is preferable.
Silica type pigments are well suited for ink jet printing due to their high absorption properties. It is believed that absorptive silica particles act like a xe2x80x9cmolecular sieve,xe2x80x9d whereby the primary particle absorbs the aqueous ink medium, but holds the dye on the surface of the particle. This results in rapid dry time and improved ink hold out.
When evaluating the characteristics of voids in silica pigments, it is important to distinguish between primary particles, secondary or agglomerated particles and associated voids. The primary particles of some silicas agglomerate, that is, groups of primary particles cluster to form secondary particles. Other silicas lack agglomerates. Further, the structure of the primary particle may vary depending on the manufacturing process. Precipitated and gel processes produce porous primary particles, while the fumed silica process produces relatively small, glassy, non-porous particles. As a result, different void structures are created. Depending on the nature of the silica, there may be voids between secondary particles, voids between primary particles within the secondary particle structure, and voids within the primary particle. For purposes of the present application, the term xe2x80x9cabsorptive silicaxe2x80x9d means a silica where the primary particles are porous and capable of absorbing water. Also for the purposes of this application, the term xe2x80x9cnon-agglomeratedxe2x80x9d refers to a silica substantially free from agglomerates, i.e., the primary particles do not exhibit a tendency to cluster into secondary particles. Gel silicas are preferred as non-agglomerated, absorptive silicas, but some precipitated and hybrid silicas may also be non-agglomerated and absorptive. Generally, fumed silicas agglomerate and are not absorptive. For a further discussion of the properties of silica minerals see Withiam, Michael C., Silica Pigments for Ink Jet Printing, article presented at 1996 TAPPI Coating Conference, Nashville, Tenn., May 21, 1996. Precipitated silicas, calcium silicates and diatomaceous earths also have good absorption capabilities, but aqueous dispersions thereof have pH values that normally range 7.0 to 9.5.
Silica gels are preferred. When dispersed, silica gels have pH values range from 3.5 to 11.0, but many are in the 4.5 to 5.5 range. The physical properties of commercially available silica gels are as follows:
Ink jet print quality and coating rheology require a special set of properties. Silicas with high surface areas, above 400 m2/g are desiccants. Desiccants are unacceptable because they will absorb moisture from the air thereby reducing ink absorptive capacity. Silica gels with low pore volumes, less than 0.5 cc/g, are undesirable, as they exhibit insufficient water absorption capacity. High pore volumes are desirable for water absorption capacity, but volumes above about 2.0 cc/g give excessively high coating viscosities. Also, larger particle sizes are preferred for having high pore volume, but particle sizes greater than 16xcexc can give poor coating rheologies. Thus, the preferred properties of the silica pigment in the coating composition of the invention are as follows:
One such pigment is Syloid 620 from Grace Davison, W. R. Grace and Co., Connecticut.
It has been found that cationized pigments hold more dye at the surface than anionic pigments. Cationic fixing agents are effective at low pH conditions, but become less effective at alkaline pH values. The type of cationic fixing agent is not critical as long as it is compatible with the other coating ingredients. The fixing agent must be effective at concentrating the dyes at the coated surface, provide satisfactory runability on a blade coater, and not adversely affect brightness. Suitable fixing agents include acrylamideacrylic polymers and their derivatives, polyamines and their derivatives, poly(ethylene oxide), and allylamine polymers. Preferably, the cationic fixing agent is a water soluble polymer having a high percentage of cationic groups such as tertiary amino or quaternary ammonium cationic groups. Water soluble, polycationic quaternary ammonium polymer (polydimethyldiallyl-ammonium chloride) is preferred in the formulation of the invention because the level of the agent may be varied substantially without flocculating the other coating ingredients. An example of this product is Lectrapel, marketed by Calgon Corporation, Water Management Division, Pittsburgh, Pa. The cationic fixing agent is added in an amount of from about 4 to about 10 bone dry parts by weight per 100 parts of pigment.
Polyvinyl alcohol (PVOH) is an acceptable binder for the coating formulation of the invention. Starches and latexes are also suitable binders and could provide satisfactory strength. Latex binders may be advantageously used in combination with polyvinyl alcohol. Some latex binders are incompatible with cationic fixing agents, however. Accordingly, if a cationic fixing agent is used, one must select a compatible latex. Also, many starches have lower binding strength than PVOH and would require excessive levels of use.
Many polyvinyl alcohols can be used, including low and medium molecular weight, partially and fully hydrolyzed. Fully hydrolyzed products are too water insoluble and give long ink dry times. Medium molecular weight products give excessive viscosities and poor rheologies. Thus, the preferred polyvinyl alcohols are low molecular weight, partially hydrolyzed. One such product is Airvol 805 from Air Products and Chemicals, Inc., Allentown, Pa.
A bright coating and one that is FDA approved for food packaging uses is desirable. Fluorescent whitening agents cannot be used for food packaging applications because they are not FDA approved. Alumina pigments are FDA approved, and are well suited whitening agents for the coating formulation of the invention. For the purposes of this application, the term xe2x80x9caluminaxe2x80x9d is used broadly to include aluminum oxide [Al2O3], aluminum trihydrate [Al(OH)3] and other conventional aluminum containing pigments. These pigments also provide some alkalinity, which is desirable for use with alkaline-stabilized ink jet dyes. Alumina pigments can be stable at both cationic and low pH conditions. Several pigments would work in the formulation, but a dry alumina that can be added directly to the coating formulation of the invention and readily dispersed in it is preferred. One such product is Martifin OL-107 marketed by Martinswerk GmbH, Bergheim, Germany. The Martifin pigment readily disperses in the coating and is compatible with the other coating ingredients. The Martifin pigment, while increasing coating solids, improves rheology at the same time. Thus, inclusion of this pigment allows application by blade coater and achieves desirable coat weights of approximately 13 g/m2 at 35% solids. This pigment also provides an acceptable brightness of 87 in a coating without fluorescent whitening agents.
For grades not requiring FDA approval, it is desirable to add a fluorescent whitening agent to the coating to increase the whiteness, brightness and blue color. While most fluorescent whitening agents would work to a certain degree, they must be stable in acid conditions and must be tolerant of the cationic coating to give optimum results. Most fluorescent whitening agents do not fall into this category. Two products, however, that do are from the Stilbene-Triazine derivatives family. Products of this type are Ciba-Geigy""s Tinopal HST and SCP liquids, available from Ciba-Geigy, Paper Dyes and Chemicals, Greensboro, N.C.
To achieve the desired coating properties and ink jet quality the coating composition is preferably made down with the following order of addition:
If polyvinyl alcohol (PVOH) is the desired binder, it must be dispersed before beginning the make down process. Preferably, the polyvinyl alcohol is cooked at 15% solids in water for 30 minutes at 95xc2x0 C. This cooking process completely disperses the polyvinyl alcohol in the water.
In the first make down step, the water for dispersion, the cationic fixing agent and cooked polyvinyl alcohol at 15% solids are added to the make down vessel. Disperser speed is then increased to add sufficient shear to disperse the alumina pigment. The disperser can be of any of the normal dispersing blades such as Cowles or Gaulin. Alumina can be rapidly added to the coating as the pigment is readily dispersed.
After dispersing the alumina pigment for 10 minutes, the silica pigment is then added. The silica pigment must be added slowly into the vortex so it can be completely wetted out and dispersed before additional pigment is added. The disperser speed must be high enough to support a vortex into which the silica is added. Silica pigment added anywhere else but into the vortex will build-up on the tank wall and harden. It will not be properly dispersed. Both the cationic agent and silica pigment should be added at the indicated points of addition. If not, the coating could have excessively high viscosities or incompatibilities and grit could result.
After the silica pigment addition is complete, the remaining polyvinyl alcohol and any additives, e.g., a fluorescent whitening agent are added. The coating is then dispersed for an additional 10-20 minutes depending on volume. The coating composition is then ready for use.
The solids content of the coating composition suspension should be above 25%, and preferably above 30% to achieve weight of at least 7.3 g/m2 with conventional coating equipment. Desirably, the percent solids should be as high as possible to reduce the energy needed to dry the coating composition on the substrate. It has been found that above about 38% solids the coating composition of the invention is difficult to smoothly apply with conventional equipment. Solids contents of 35-38% provides desirable coat weights of about 13 g/m2.
The ink jet recording medium of the present invention comprises a substrate with the coating composition of the invention applied thereto. The substrate may comprise a variety of types of paper webs or, plastic substrates such as mylar. Paper webs may include groundwood-free sheets, groundwood sheets or a combination thereof. The basis weight of acceptable paper substrates may vary greatly, from very light Bible papers having basis weight of about 32 g/m2 to heavy, specialty papers having basis weight of 450 g/m2 or more. Paper substrates may be uncoated, size press coated or precoated, and the paper may be machine-glazed or machine finished. Depending on the nature of the substrate, a precoating or other treatment may be useful to reduce porosity, or to provide a better bonding surface for the subsequent coating, or to better prevent migration of the subsequent coating into the web. Preferably, rosin or other sizing is added to achieve 40 or less g/m2/min Cobb sizing, to reduce penetration of liquid into the web.
One or both sides of the paper web may be precoated with size solution to provide brightness and color and to provide sufficient holdout for the final coating. The precoating is applied in a conventional manner and may contain conventional pigments, binders and sizing agents. Preferably, the TAPPI brightness is 85 or greater, and the TAPPI xe2x80x9cbxe2x80x9d color is equal to or less than 2. If the subsequently applied ink-receiving coat will be applied to only one side of the web, a lightweight coating may also be applied to the other side of the web to minimize potential sheet curl.
The precoat comprises conventional pigments such as clay, titanium dioxide, calcium carbonate and others well known to those skilled in the art. The binders may comprise starch, soy protein, latex and the like. A sizing agent may be employed such as rosin, starch and other known sizing agents. The base web is preferably sized at values less than 40 g/m2/min Cobb size, and the coat weight is in the order of about 3.2 to about 8.1 g/m2. Cobb size is a standard test to determine the amount of water absorbed during contact with the web and is measured in grams per square meter per minute.
A preferred substrate for cut size ink jet papers comprises a low ash base stock made square (having similar CD and MD physical properties), having a basis weight of between 74-119 g/m2. The base stock is size coated with a light starch and pigment coating and dried prior to application of the ink receptive coating.
A preferred substrate suitable for ink jet label paper comprises 48-65 g/m2 base stock with a wet strength resin in the base sheet to prevent cockle. The machine-glazed side of the sheet is coated with a pigment latex coating for curl control. The ink receptive coating is applied to the back side of the sheet.
After the web has been dried, the ink receptive coating of the invention is preferably applied over at least one side using a conventional coater, and then is dried. The desired coat weight is at least 7.3 g/m2 and preferably 8-16.5 g/m2. If the weight is significantly below 7.3 g/m2, the resulting paper will exhibit less than desirable print quality and excessive ink penetration. After drying, the ink receptive coating layer will have a thickness of at least 8xcexc and preferably from about 8 to about 12xcexc. The pigments in the coating provide an absorptive capacity for the liquid component of the ink to be applied, and the thickness of the coating layer is correlated to the absorption rate and hence ink drying time.
Depending on resolution of the printer, the size of the dots to be printed ranges in the order of 75 to 160xcexc. Ink jet printing of 1000 to 1200 dots per inch, when available, will require dots having a diameter of down to 40xcexc or less. The present invention contemplates the user of various binders and sizing agents, depending on the resolution needed for a printer. The binder level and sizing agents contribute to the control of dot diameter and other properites.